The present invention relates to a muffler for an exhaust gas from an internal combustion engine (hereinafter simply referred to as a muffler for an exhaust gas) and more particularly aims to widen the range of sound-deadening performance of the muffler.
FIGS. 1(a) and 1(b) show schematic diagrams of the conventional muffler for an exhaust gas. In the drawings, the reference numeral 1 designates an inlet pipe, 2 a casing of a muffler for exhaust gas, 3 a perforated pipe made of punched metal, 4 an outlet pipe, and 5 a sound-absorbing material filled in a space formed by the perforated pipe 3 and the casing 2. Usually, fibrous sound-absorbing materials, such as glass or rock wool, are used as the sound-absorbing material. The inlet pipe 1, the perforated pipe 3, and the outlet pipe 4 are disposed serially to constitute an exhaust gas passage 6. In the thus arranged muffler for an exhaust gas, the exhaust gas entering the inlet pipe 1 passes through the perforated pipe 3 and the outlet pipe 4 and is scattered into the air. The sound accompanying the exhaust gas propagates into slender interstices in the sound-absorbing material 5, causing acoustic energy of the exhaust gas sound to be converted into heat energy by a viscosity effect, so that the sound is deadened.
A disadvantage of such conventional mufflers is that the sound-deadening performance deteriorates remarkably with age. There are several reasons for this. First, the aperture portions in the sound-absorbing material become clogged because combustion remnants (such as soot, tar) in the exhaust gas enter the aperture portions and adhere thereto. Second, since the sound-absorbing material is fibrous, the fabric may be scattered by the exhaust gas. Third, since the sound-absorbing material completely fills the casing, the effect of its heat insulation properties is large and the interior of the casing will have a relatively low temperature, causing steam in the exhaust gas to condense. The condensed steam combines with a sulfur dioxide gas, or the like, to form a strongly acidified compound, thereby corroding the casing and permitting the sound to be radiated in the air therefrom.
Applicants have made a study for the purpose of avoiding the problem of age deterioration in conventional mufflers as described above. As a result, Applicants have found that the aforementioned second and third problems could be solved in the manner illustrated in FIGS. 2(a) and 2(b). The exhaust gas is prevented from scattering by using a metallic porous body 7 as a sound-absorbing material, and the corrosion problem of the casing is solved by providing a rear air layer 8 between the metallic porous body 7 and the casing 2 to avoid a large temperature reduction in the casing 2 in order to suppress generation of condensed water. The reference numeral 71 designates that the sound-absorbing body is porous. The sound-absorbing material (the metallic porous body) is quite hard and may be a frame member.
While the device of FIGS. 2(a) and 2(b) solves the second and third causes of age deterioration, it does not diminish the first cause, namely, clogging of openings, which is the main cause of age deterioration. Applicants have found that prevention of clogging of the sound-absorbing material can be accomplished by forming an airtight thin film on the surface of the sound-absorbing material with which an exhaust gas comes into contact to thereby block the flow of the gas into the material. In general, the forming of such a thin film reduces the propagation of the sound wave itself into the sound-absorbing material, thereby deteriorating the sound-absorbing properties of the muffler. However, Applicants have found that the sound-absorbing performance of such a muffler can be improved relative to a muffler with no thin film, by properly adjusting the thickness of the thin film and the aperture rate in the sound-absorbing material. That is, it is possible to increase the sound absorption in the frequency range where high sound absorption is desired above that of a muffler having only the sound-absorbing material per se with no thin film, by setting an intrinsic value of a machine-acoustic impedance system constituted by the thin film, the apertures of the sound-absorbing material, etc.
FIG. 3 is a graph of experimental results illustrating the latter improvement in sound absorption. Curves A and B represent the absorption of the same sound-absorbing porous material, the only difference being that the device resulting in curve B was provided with a 10 .mu.m thin film of a nickel-chrome alloy.
To provide the thin film onto the surface of the sound-absorbing member, methods of applying, adhering, bonding, integral molding, sandwiching, etc. are used. Whichever method is employed, it becomes fundamentally possible to prevent the clogging due to the combustion remnants from occurring in the sound-absorbing member and to improve the sound-absorbing rate.
However, an additional problem has been discovered as a result of actually mounting a sound-absorbing device using the sound-absorbing material with the thin film in an internal combustion engine. Since the thin film prevents the gases from passing through the sound-absorbing material, a pressure difference is produced between the surface of the sound-absorbing member in contact with the exhaust gases and the outer surface of the sound-absorbing material. The pressure difference exerts a large amount of tension upon the thin film, thereby increasing the film hardness. Thus, the vibration response property of the film is lowered to thereby cause deterioration in the sound-absorbing rate. Moreover, if the pressure difference becomes too large, the film may be destroyed. Applicants have found, as a countermeasure therefor, a method of reducing the pressure difference by providing a pressure balancing opening, which is formed by cutting away a part of each of the sound-absorbing material and the thin film. That is, as shown in FIGS. 4(a) and 4(b), a thin film 9 is formed between a perforated pipe 3 and a metallic porous body 7, and a pressure balance opening 10 is formed by cutting away a part of each of the metallic porous body 7 and the thin film 9. In this arrangement, although an exhaust gas is scattered out in the air through an exhaust gas passage constituted by an inlet pipe 1, the perforated pipe 3 and an outlet pipe 4, a part of the exhaust gas is allowed to flow into or out of a casing 2 through the balance opening 10, so that the pressure difference at the opposite sides of the thin film 9 may be reduced. This prevents the thin film from being destroyed and allows the thin film to act effectively to increase the sound-absorbing properties of the muffler. A muffler of this type is described in U.S. application Ser. No. 531,894, filed July 5, 1983.
As described above, in an exhaust gas muffler constituted by a thin film, a sound-absorbing material and a balance opening hole, as illustrated in FIGS. 4(a) and 4(b), the sound-absorbing properties of the sound-absorbing material are considerably improved over that of a sound-absorbing material with no thin film. However, the sound-absorbing properties decrease in a frequency band below 200 Hz.